A vacuum insulated glass (VIG) unit comprises two sheets of glass with at least one vacuum space in between them that is at a pressure less than atmospheric pressure. Separation of the glass sheets and maintenance of the vacuum space is accomplished by some sort of physical spacer system in between the glass sheets that resists the compressive load of atmospheric pressure and contributes to the prevention of collapse of the vacuum space. Around the periphery of the unit is at least one edge seal that seals the vacuum space from the atmosphere. The vacuum space reduces heat conduction and convection between the glass sheets. The spacer system, which conducts an amount of heat between the glass sheets that depends in part on the thermal conductivity of its materials, can tend to negate the high insulating value of the vacuum. A rigid or somewhat inflexible edge seal such as glass frit or metal may result in unacceptably high stress in a VIG unit if the temperature difference between the inner and outer glass sheets becomes too large as might occur in cold climates, so that the use of these materials for an edge seal can limit acceptable climates, the size of a unit, or both.
Current state of the art VIG units are evacuated once and permanently sealed at time of assembly. Those that utilize glass frit edge seals and discrete metal spacers maintain their service pressures only because the glass sheets, glass edge seals, and metal spacers can achieve extremely low outgassing rates in a high temperature bakeout at time of assembly.